To develop therapeutic and vaccine strategies to combat HIV-1, the cause of AIDS, it is essential to understand the cell-mediated immune response to this agent. To pursue this goal, murine model systems for immunization, isolation of antigen-specific T cells, and in vitro analysis of T cells specific for HIV-1 proteins have been devised. Structure- function studies on a class II-restricted determinant have revealed how a small subset of residues controls binding to MHC molecules, how additional residues regulate the epitopic structure necessary for T cell recognition, and how it may be possible to improve the potency of vaccine antigens by removal of interfering groups in the natural antigen. Restimulation in vitro of spleen cells from H-2d mice immunized with recombinant vaccinia virus containing the HIV-1 (IIIB, MN, or RF) gp160 gene elicited CD8+ cytolytic T cells specific for amino acids 315-329. All responses were found in H-2d mice, and the CTL elicited were H-2Dd restricted, but the CTL produced were highly specific and did not cross-react on target cells incubated with peptides corresponding to the sequences of the different isolates. To understand the relationship among these and other HIV isolates in terms of CTL specificity, variant peptides representing either natural isolates or synthetic combinations of residues from the natural sequences were prepared and used as immunogens or target antigens. These studies showed that the CTL responses to HIV fall into groups related by the chemical nature of key epitopic residues in the inducing peptide. Further, alternation in the peptide structure between immunization and boost broadened the CTL response and suggests a strategy for immunization in man.